Hilchos Lashon Hara chapter 8, section 9

Know also that it is forbidden to disparage and insult the dead. The poskim write that the early sages made an enactment and a ban not to speak evil of the dead. This applies even if the deceased is unlearned. If he is a Torah scholar disparaging him is an enormous sin that deserves excommunication as the Yoreh Deáh rules (243:7). The prohibition against disparaging a Torah scholar applies even to disparaging him, and how much more so to disparaging his Torah.

The Chofetz Chaim

Rav Yisroel Meir Hakohen of Radin zt"l known as the Chofetz Chaim, was one of the most famous rabbis before the Holocaust.
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